WEBrickを立ち上げようとしたらエラーが出た。

shell

$ bin/rails s
bin/rails:6: warning: already initialized constant APP_PATH
bin/rails:6: warning: previous definition of APP_PATH was here
Usage: rails COMMAND [ARGS]

The most common rails commands are:
 generate    Generate new code (short-cut alias: "g")
 console     Start the Rails console (short-cut alias: "c")
 server      Start the Rails server (short-cut alias: "s")
 dbconsole   Start a console for the database specified in config/database.yml
             (short-cut alias: "db")
 new         Create a new Rails application. "rails new my_app" creates a
             new application called MyApp in "./my_app"

In addition to those, there are:
 application  Generate the Rails application code
 destroy      Undo code generated with "generate" (short-cut alias: "d")
 plugin new   Generates skeleton for developing a Rails plugin
 runner       Run a piece of code in the application environment (short-cut alias: "r")

All commands can be run with -h (or --help) for more information.

このエラーメッセージだけでは検討つかなかったが、
MySQLのlibmysqlclientが読み込めていなかったのが原因だった。

以下のコマンドでシンボリックリンクを貼ったら、
正常にサーバが起動するようになった。

shell

sudo ln -s /usr/local/mysql/lib/libmysqlclient.18.dylib /usr/lib/libmysqlclient.18.dylib

参考

rails server bin/rails:6: warning: already initialized constant APP_PATH error
http://stackoverflow.com/questions/23402820/rails-server-bin-rails6-warning-already-initialized-constant-app-path-error